Cascaded multi-level inverter system and modulation method thereof, and controller

ABSTRACT

A cascaded multi-level inverter system, a modulation method and a controller for the same are provided. The method includes performing a maximum power point tracking control based on a voltage signal and a current signal of each DC source and a voltage signal and a current signal of the power grid obtained by sampling, calculating a first modulation signal for suppressing power imbalance, and outputting the first modulation signal to each inverter unit; and calculating, based on the calculated reactive current instruction value, the calculated active current instruction value, and a current signal of the reactive compensation device obtained by sampling, a second modulation signal for causing an output power factor of the cascaded multi-level inverter system to be 1, and outputting the second modulation signal to the reactive compensation device.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the priority to Chinese PatentApplication No. 201610700663.3, entitled “CASCADED MULTI-LEVEL INVERTERSYSTEM AND MODULATION METHOD THEREOF, AND CONTROLLER”, filed on Aug. 22,2016 with the State Intellectual Property Office of the People'sRepublic of China, which is incorporated herein by reference in itsentirety.

TECHNICAL FIELD

The present disclosure relates to the technology field of invertersystem modulation, and in particular to a cascaded multi-level invertersystem, a modulation method and a controller for the same.

BACKGROUND

At present, cascaded multi-level technology, as a kind of maturetechnology, is well developed in fields of motor drive, medium voltageSVG and the like. In recent years, as a photovoltaic module is anindependent direct current (DC) source, the cascaded multi-leveltechnology is also widely used day by day in photovoltaic field. Acascaded multi-level inverter system can track a maximum power point ofeach photovoltaic module, and has a relative advantage in efficiency asa multi-level topology. The topology structure of the cascadedmulti-level inverter system is shown in FIG. 1. However, there are somedifficulties in applications of the cascaded multi-level inverter systemin the photovoltaic field. For example, when the photovoltaic modulefails, or is shielded, or is mismatched seriously, power of thephotovoltaic module is unbalanced in different degrees, thereby leadingto a problem of a low modulation voltage of the whole system and amodulation voltage saturation of an inverter module, and furtherseriously impacting working stability of a photovoltaic system which mayresult in a shutdown of the system for security.

To suppress the foregoing situations of power imbalance, a method ofreactive power compensation is applied commonly in the prior art. Themethod is to inject capacitive or inductive reactive power into anoutput current, and compensate a modulation voltage to realize a stableoperation of the whole system.

The above method of reactive power compensation to suppress the powerimbalance may suppress the power imbalance with different degrees, butcapacitive or inductive reactive power may also be injected into thepower grid in the method, which is not allowed by the power grid.

SUMMARY

The present disclosure provides a cascaded multi-level inverter system,a modulation method and a controller for the same, to solve the problemof injecting reactive current into the power grid in the conventionaltechnology.

The above object is achieved through the following technical solutions:

A modulation method for a cascaded multi-level inverter system isprovided, which is applied to a controller for the cascaded multi-levelinverter system. The cascaded multi-level inverter system includes areactive compensation device and multiple inverter units connected withthe controller, the reactive compensation device is connected with apower grid, and the inverter units are connected with multiple DCsources respectively. The modulation method includes:

performing a maximum power point tracking control based on a voltagesignal and a current signal of each DC source and a voltage signal and acurrent signal of the power grid obtained by sampling, calculating afirst modulation signal for suppressing power imbalance, and outputtingthe first modulation signal to each inverter unit;

calculating a reactive compensation current component based on thevoltage signal and the current signal of each DC source and the voltagesignal of the power grid obtained by sampling;

calculating a reactive current instruction value which is equal in sizeand opposite in direction to the reactive compensation current componentbased on the reactive compensation current component;

calculating an active current instruction value based on a DC-sidevoltage set value signal of the reactive compensation device, and aDC-side voltage signal of the reactive compensation device obtained bysampling; and

calculating, based on the reactive current instruction value, the activecurrent instruction value, and a current signal of the reactivecompensation device obtained by sampling, a second modulation signal forcausing an output power factor of the cascaded multi-level invertersystem to be 1, and outputting the second modulation signal to thereactive compensation device.

Preferably, before the process of outputting the second modulationsignal to the reactive compensation device, the method further includes:

calculating a harmonic current component based on the voltage signal andthe current signal of the power grid obtained by sampling; and

calculating, as an output, a second modulation signal for compensatingtotal harmonic distortion, based on the harmonic current component andthe second modulation signal for causing an output power factor of thecascaded multi-level inverter system to be 1.

Preferably, the process of performing a maximum power point trackingcontrol based on a voltage signal and a current signal of each DC sourceand a voltage signal and a current signal of the power grid obtained bysampling, calculating a first modulation signal for suppressing powerimbalance, includes:

performing a maximum power point tracking calculation and a maximumpower point tracking control based on the voltage signal and the currentsignal of each DC source obtained by sampling, and acquiring a powerinstruction value of each inverter unit;

calculating a power gird current instruction value based on the powerinstruction value of each inverter unit, the reactive compensationcurrent component, and the voltage signal of the power grid obtained bysampling;

calculating a modulation voltage instruction value based on the powergrid current instruction value, and the current signal of the power gridobtained by sampling;

dividing the modulation voltage instruction value into an activemodulation signal and a reactive modulation signal; and

distributing the active modulation signal and the reactive modulationsignal according to an active power distribution principle and areactive power distribution principle respectively, and calculating, bymeans of vector synthesis, the first modulation signal to be outputtedto each inverter unit.

A controller for a cascaded multi-level inverter system is provided,which is applied to a reactive compensation device and multiple inverterunits of the cascaded multi-level inverter system. The reactivecompensation device is connected with a power grid, and the multipleinverter units are connected with multiple DC sources respectively. Thecontroller includes:

a first modulation module configured to perform a maximum power pointtracking control based on a voltage signal and a current signal of eachDC source, and a voltage signal and a current signal of the power gridobtained by sampling, calculate a first modulation signal forsuppressing power imbalance, and output the first modulation signal toeach inverter unit; and

a second modulation module configured to calculate a reactivecompensation current component based on the voltage signal and thecurrent signal of each DC source and the voltage signal of the powergrid obtained by sampling; calculate a reactive current instructionvalue which is equal in size and opposite in direction to the reactivecompensation current component based on the reactive compensationcurrent component; calculate an active current instruction value basedon a DC-side voltage set value signal of the reactive compensationdevice, and a DC-side voltage signal of the reactive compensation deviceobtained by sampling; and calculate, based on the reactive currentinstruction value, the active current instruction value, and a currentsignal of the reactive compensation device obtained by sampling, asecond modulation signal for causing an output power factor of thecascaded multi-level inverter system to be 1, and output the secondmodulation signal to the reactive compensation device.

Preferably, the second modulation module are further configured to:

calculate a harmonic current component based on the voltage signal andthe current signal of the power grid obtained by sampling; and

calculate, as an output, a second modulation signal for compensatingtotal harmonic distortion, based on the harmonic current component andthe second modulation signal for causing an output power factor of thecascaded multi-level inverter system to be 1.

Preferably, the first modulation module further includes:

a first controlling module configured to perform a maximum power pointtracking calculation and a maximum power point tracking control based onthe voltage signal and the current signal of each DC source obtained bysampling, and acquire a power instruction value of each inverter unit;

a first calculation module configured to calculate a power gird currentinstruction value based on the power instruction value of each inverterunit, the reactive compensation current component, and the voltagesignal of the power grid obtained by sampling;

a second calculation module configured to calculate a modulation voltageinstruction value based on the power grid current instruction value, andthe current signal of the power grid obtained by sampling;

a third calculation module configured to divide the modulation voltageinstruction value into an active modulation signal and a reactivemodulation signal; and

a fourth calculation module configured to distribute the activemodulation signal and the reactive modulation signal according to anactive power distribution principle and a reactive power distributionprinciple respectively, and calculate, by means of vector synthesis, thefirst modulation signal to be outputted to each inverter unit.

A cascaded multi-level inverter system is provided, which includes areactive compensation device, multiple inverter units, and a controllerin which any of the foregoing modulation methods for the cascadedmulti-level inverter system is applied,

the reactive compensation device is connected with a power grid;

the multiple inverter units are connected with multiple DC sourcesrespectively; and

the reactive compensation device is a voltage-type reactive compensationcircuit, a current-type reactive compensation circuit or a switchclamping type three-level reactive compensation circuit.

Preferably, the voltage-type reactive compensation circuit includes anH-bridge inverter module, a first capacitor and a first inductor;

the first capacitor is connected between two input terminals of theH-bridge inverter module;

one end of the inductor is connected with one output terminal of theH-bridge inverter module; and

the other end of the inductor and the other output terminal of theH-bridge inverter module are connected with the power grid respectively.

Preferably, the current-type reactive compensation circuit includes anH-bridge inverter module and a second inductor;

the second inductor is connected between two input terminals of theH-bridge inverter module; and

two output terminals of the H-bridge inverter module are connected withthe power grid respectively.

Preferably, the voltage-type compensation circuit includes an H-bridgeinverter module, a second capacitor, a third capacitor, a firstswitching transistor, a second switching transistor and a thirdinductor;

the second capacitor and the third capacitor are connected in seriesbetween two input terminals of the H-bridge inverter module;

the series connection point between the second capacitor and the thirdcapacitor is connected, through the first switching transistor and thesecond switching transistor which are connected in anti-series, with oneoutput terminal of the H-bridge inverter module and one end of the thirdinductor;

the other end of the third inductor and the other output terminal of theH-bridge inverter module are connected with the power grid respectively.

With the modulation method for the cascaded multi-level inverter systemprovided in the present disclosure, firstly, a maximum power pointtracking control is performed based on a voltage signal and a currentsignal of each DC source and a voltage signal and a current signal ofthe power grid obtained by sampling, a first modulation signal forsuppressing power imbalance is calculated, and the first modulationsignal is outputted to each inverter unit, thereby realizing the maximumpower point tracking control and the power imbalance suppression. Then,based on the calculated reactive current instruction value, thecalculated active current instruction value, and a current signal of areactive compensation device obtained by sampling, a second modulationsignal is calculated for causing an output power factor of the cascadedmulti-level inverter system to be 1, and the second modulation signal isoutputted to the reactive compensation device. The reactive currentinstruction value is equal in size and opposite in direction to areactive compensation current component, thereby making an output powerfactor of the cascaded multi-level inverter system is 1 to meetrequirements of the power grid.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompany drawings used in the description of the embodiments willbe described briefly as follows, so that the technical solutions basedon the embodiments of the present disclosure will become more apparent.It is clear that the accompany drawings in the following description areonly some embodiments of the present disclosure. For those skilled inthe art, other accompany drawings may be obtained based on theseaccompany drawings without any creative work.

FIG. 1 is a schematic structural diagram of a cascaded multi-levelinverter system in the prior art;

FIG. 2 is a schematic circuit diagram of a cascaded multi-level invertersystem according to an embodiment of the present disclosure;

FIG. 3 is a flow diagram of a modulation method for a cascadedmulti-level inverter system provided according to an embodiment of thepresent disclosure;

FIG. 4 is a flow diagram of a modulation method for a cascadedmulti-level inverter system provided according to an embodiment of thepresent disclosure;

FIG. 5 is a flow diagram of a modulation method for a cascadedmulti-level inverter system provided according to an embodiment of thepresent disclosure;

FIG. 6 is a schematic structural diagram of a controller for a cascadedmulti-level inverter system according to an embodiment of the presentdisclosure;

FIG. 7 is a schematic structural diagram of a controller for a cascadedmulti-level inverter system according to an embodiment of the presentdisclosure;

FIG. 8 is a schematic circuit diagram of a reactive compensation deviceaccording to an embodiment of the present disclosure;

FIG. 9 is a schematic circuit diagram of a reactive compensation deviceaccording to an embodiment of the present disclosure; and

FIG. 10 is a schematic circuit diagram of a reactive compensation deviceaccording to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

For explaining objects, technical solutions and advantages of thedisclosure more clearly, embodiments of the disclosure are furtherdescribed hereinafter in conjunction with drawings.

The present disclosure provides a modulation method for a cascadedmulti-level inverter system to solve the problem of injecting reactivepower into the power grid in the prior art.

Specifically, the modulation method for the cascaded multi-levelinverter system is applied to a controller for a cascaded multi-levelinverter system. The cascaded multi-level inverter system, as shown inFIG. 2, includes a reactive compensation device and multiple inverterunits connected with the controller (taking an H-bridge module as anexample in FIG. 2). The reactive compensation device is connected with apower grid, and the inverter units are connected with multiple DCsources respectively. The modulation method for the cascaded multi-levelinverter system, as shown in FIG. 3, includes steps S101 to S105.

In step S101, a maximum power point tracking control is performed basedon a voltage signal and a current signal of each DC source and a voltagesignal and a current signal of the power grid obtained by sampling, afirst modulation signal is calculated for suppressing power imbalance,and the first modulation signal is outputted to each inverter unit.

In step S102, a reactive compensation current component is calculatedbased on the voltage signal and the current signal of each DC source andthe voltage signal of the power grid obtained by sampling.

In step S103, a reactive current instruction value is calculated basedon the reactive compensation current component. The reactive currentinstruction value is equal in size and opposite in direction to thereactive compensation current component.

In step S104, an active current instruction value is calculated based ona DC-side voltage set value signal of the reactive compensation deviceand a DC-side voltage signal of the reactive compensation deviceobtained by sampling.

In step S105, based on the reactive current instruction value, theactive current instruction value, and a current signal of the reactivecompensation device obtained by sampling, a second modulation signal iscalculated for causing an output power factor of the cascadedmulti-level inverter system to be 1, and the second modulation signal isoutputted to the reactive compensation device.

By taking step S101 to control each inverter unit, suppress powerimbalance, and realize stable operation of the whole cascadedmulti-level inverter system, each DC source (taking a photovoltaicmodule as an example in FIG. 2) achieves MPPT (Maximum Power PointTracking), and the power grid may be injected with reactive current forsuppressing power imbalance.

Then by taking steps S102 to S105 to control the reactive compensationdevice and eliminate the reactive current for suppressing powerimbalance by means of a reactive current instruction value which isequal in size and opposite in direction to the reactive compensationcurrent component, an output power factor of the cascaded multi-levelinverter system becomes 1, for meeting requirements of the power grid.

In practice, the sequence between step S101 and steps S102 to S105 isnot defined, but depends on the specific application environment, andFIG. 3 is only an example. Any solutions which can realize MPPT controlof each DC source, power imbalance suppression and an output powerfactor of the cascaded multi-level inverter system being 1 fall withinthe scope of the present disclosure.

With the modulation method for the cascaded multi-level inverter systemprovided in embodiments of the present disclosure, by taking theforegoing steps, a photovoltaic module is still capable of being in aMPPT working state even in situations that the photovoltaic module isserious unbalanced, for example, when a part of the photovoltaic modulefails, or is shielded, or is damaged. The system can realize stablepower generation; and the output power factor is 1, which meetsrequirements of the power grid, thereby solving the problem in the priorart.

It is important to note that, there are also solutions to suppress powerimbalance and realize the system stable operation by means of the MPPTworking area, which are only capable of suppressing a part of relativemild imbalance. And a cost for the suppressing is a considerable loss ofpower generation, which may cause great economic losses.

With the modulation method for the cascaded multi-level inverter systemprovided in embodiments of the present disclosure, each DC source(taking a photovoltaic module as an example in FIG. 2) is able toperform MPPT, thus realizing the function of normal MPPT grid-connectedpower generation, the normal electric energy production is ensured, andthe economic losses in the prior art are avoided.

Another modulation method for a cascaded multi-level inverter system isprovided in another preferred embodiment of the present disclosure, asshown in FIG. 4, which includes steps S201 to S206.

In step S201, a maximum power point tracking control is performed basedon a voltage signal and a current signal of each DC source and a voltagesignal and a current signal of the power grid obtained by sampling, afirst modulation signal is calculated for suppressing power imbalance,and the first modulation signal is outputted to each inverter unit.

In step S202, a reactive compensation current component is calculatedbased on the voltage signal and the current signal of each DC source andthe voltage signal of the power grid obtained by sampling.

In step S203, a reactive current instruction value is calculated basedon the reactive compensation current component. The reactive currentinstruction value is equal in size and opposite in direction to thereactive compensation current component.

In step S204, an active current instruction value is calculated based ona DC-side voltage set value signal of the reactive compensation deviceand a DC-side voltage signal of the reactive compensation deviceobtained by sampling.

In step S205, a harmonic current component is calculated based on thevoltage signal and the current signal of the power grid obtained bysampling.

In step S206, based on the harmonic current component, the reactivecurrent instruction value, the active current instruction value, and acurrent signal of the reactive compensation device obtained by sampling,a second modulation signal is calculated for causing an output powerfactor of the cascaded multi-level inverter system to be 1 andcompensating total harmonic distortion, and the second modulation signalis outputted to the reactive compensation device.

By taking steps S205 and S206, the reactive compensation device may alsofunction as a harmonic current compensation device to compensate THD(Total Harmonic Distortion) of an input current, thereby decreasing THDof an output current; and facilitating the application of the cascadedmulti-level inverter system.

In practice, the sequence between step S201 and steps S202 to S206 isnot defined, but depends on the specific application environment, andFIG. 4 is only an example. Any solutions which can realize MPPT controlof each DC source, power imbalance suppression, an output power factorof the cascaded multi-level inverter system being 1 and reduction of THDof an output current fall within the scope of the present disclosure.

Another modulation method for a cascaded multi-level inverter system isprovided in another preferred embodiment of the present disclosure, asshown in FIG. 5, which includes steps S301 to S310.

In step S301, a maximum power point tracking calculation and a maximumpower point tracking control are performed based on the voltage signaland the current signal of each DC source obtained by sampling, and apower instruction value of each inverter unit is acquired.

In step S302, a power gird current instruction value is calculated basedon the power instruction value of each inverter unit, the reactivecompensation current component, and the voltage signal of the power gridobtained by sampling.

In step S303, a modulation voltage instruction value is calculated basedon the power grid current instruction value, and the current signal ofthe power grid obtained by sampling.

In step S304, the modulation voltage instruction value is divided intoan active modulation signal and a reactive modulation signal.

In practice, the dividing of the modulation voltage instruction valuemay be performed in combination with a power factor angle and the like,which is not defined here but depends on the specific applicationenvironment.

In step S305, the active modulation signal and the reactive modulationsignal are distributed according to an active power distributionprinciple and a reactive power distribution principle respectively, thefirst modulation signal to be outputted to each inverter unit iscalculated by means of vector synthesis, and the first modulation signalis outputted to each inverter unit.

In step S306, a reactive compensation current component is calculatedbased on the voltage signal and the current signal of each DC source andthe voltage signal of the power grid obtained by sampling.

In step S307, a reactive current instruction value is calculated basedon the reactive compensation current component. The reactive currentinstruction value is equal in size and opposite in direction to thereactive compensation current component.

In step S308, an active current instruction value is calculated based ona DC-side voltage set value signal of the reactive compensation deviceand a DC-side voltage signal of the reactive compensation deviceobtained by sampling.

In step S309, a harmonic current component is calculated based on thevoltage signal and the current signal of the power grid obtained bysampling.

In step S310, based on the harmonic current component, the reactivecurrent instruction value, the active current instruction value, and acurrent signal of the reactive compensation device obtained by sampling,a second modulation signal is calculated for causing an output powerfactor of the cascaded multi-level inverter system to be 1 andcompensating total harmonic distortion, and the second modulation signalis outputted to the reactive compensation device.

With the specific method for controlling each inverter unit provided inthe steps S301 to S305, a photovoltaic module is still capable of beingin a MPPT working state even in situations that the photovoltaic moduleis serious unbalanced, for example, when a part of the photovoltaicmodule fails, or is shielded, or is damaged, thereby realizing thesystem generate power stably.

Of course, solutions for controlling each DC source to be in a MPPTworking state and for suppressing power imbalance are not limited tothese embodiments and may be selected and varied based on specificapplication environment. Any solutions which can realize MPPT of each DCsource and suppress power imbalance fall within the scope of the presentdisclosure.

Another embodiment of the present disclosure provides a controller for acascaded multi-level inverter system, which is applied to a reactivecompensation device and multiple inverter units of the cascadedmulti-level inverter system. The reactive compensation device isconnected with a power grid, and the multiple inverter units areconnected with multiple DC sources respectively. The controller for thecascaded multi-level inverter system, as shown in FIG. 6, includes afirst modulation module 101 and a second modulation module 102.

The first modulation module 101 is configured to perform a maximum powerpoint tracking control based on a voltage signal (v_(pv1), v_(pv2) . . .v_(pvn)) and a current signal (i_(pv1), i_(pv2) . . . i_(pvn)) of eachDC source and a voltage signal v_(s) of the power grid and a currentsignal i_(s) of the power grid obtained by sampling, calculate a firstmodulation signal (v_(H1)*, v_(H2)* . . . v_(Hn)*) for suppressing powerimbalance, and output the first modulation signal (v_(H1)*, v_(H2)* . .. v_(Hn)*) to each inverter unit.

The second modulation module 102 is configured to calculate a reactivecompensation current component i_(sq)* based on the voltage signal(v_(pv1), v_(pv2) . . . v_(pvn)) and the current signal (i_(pv1),i_(pv2) . . . i_(pvn)) of each DC source and the voltage signal v_(s) ofthe power grid obtained by sampling, calculate a reactive currentinstruction value which is equal in size and opposite in direction tothe reactive compensation current component based on the reactivecompensation current component i_(sq)* , calculate an active currentinstruction value i_(rsq)* based on a DC-side voltage set value signalv_(rdc)* of the reactive compensation device and a DC-side voltagesignal v_(rdc) of the reactive compensation device obtained by sampling,and calculate, based on the reactive current instruction value, theactive current instruction value i_(rsq)* and a current signal i_(rs) ofthe reactive compensation device obtained by sampling, a secondmodulation signal v_(r)* for causing an output power factor of thecascaded multi-level inverter system to be 1, and output the secondmodulation signal v_(r)* to the reactive compensation device.

With the controller for the cascaded multi-level inverter systemprovided in embodiments of the present disclosure, based on theforegoing theory, a photovoltaic module is still capable of being in aMPPT working state even in situations that the photovoltaic module isserious unbalanced, for example, when a part of the photovoltaic modulefails, or is shielded, or is damaged. The system can realize stablepower generation; and the output power factor is 1, which meetsrequirements of the power grid, thereby solving the problem in the priorart.

Preferably, a second modulation module 102 is further configured to:

calculate a harmonic current component i_(sh)* based on the voltagesignal v_(s) of the power grid and the current signal is of the powergrid obtained by sampling; and

calculate, as an output, a second modulation signal v_(r)* forcompensating total harmonic distortion, based on the harmonic currentcomponent i_(sh)* and the second modulation signal for causing an outputpower factor of the cascaded multi-level inverter system to be 1.

With the controller for the cascaded multi-level inverter systemprovided in embodiments of the present disclosure, the reactivecompensation device may also function as a harmonic current compensationdevice to compensate THD of an input current; and facilitating theapplication of the cascaded multi-level inverter system.

Specifically, as shown in FIG. 7, a first modulation module 101 includesa first controlling module, a first calculation module, a secondcalculation module, a third calculation module and a fourth calculationmodule.

The first controlling module is configured to perform a maximum powerpoint tracking calculation and a maximum power point tracking controlbased on the voltage signal (v_(pv1), v_(pv2) . . . v_(pvn)) and thecurrent signal (i_(pv1), i_(pv2) . . . i_(pvn)) of each DC sourceobtained by sampling, and acquire a power instruction value (P₁*, P₂* .. . P_(n)*) of each inverter unit.

The first calculation module is configured to calculate a power girdcurrent instruction value i_(s)* based on the power instruction value(P₁*, P₂* . . . P_(n)*) of each inverter unit, the reactive compensationcurrent component i_(sq)*, and the voltage signal v_(s) of the powergrid obtained by sampling.

The second calculation module is configured to calculate a modulationvoltage instruction value v_(H)* based on the power grid currentinstruction value i_(s)*, and the current signal i_(s) of the power gridobtained by sampling.

The third calculation module i_(s) configured to divide the modulationvoltage instruction value v_(H)* into an active modulation signalv_(HP)* and a reactive modulation signal v_(HQ)*.

The fourth calculation module i_(s) configured to distribute the activemodulation signal v_(HP)* and the reactive modulation signal v_(HQ)*according to an active power distribution principle and a reactive powerdistribution principle respectively, and calculate, by means of vectorsynthesis, the first modulation signal (v_(H1)*, v_(H2)* . . . v_(Hn)*)to be outputted to each inverter unit.

A second modulation module 102 includes a fifth calculation module, asixth calculation module, a seventh calculation module and an eighthcalculation module.

The fifth calculation module i_(s) configured to calculate a reactivecompensation current component i_(sq)* based on a voltage signal(v_(pv1), v_(pv2) . . . v_(pvn)) and a current signal (i_(pv1), i_(pv2). . . i_(pvn)) of each DC source and the voltage signal v_(s) of thepower grid obtained by sampling.

The sixth calculation module i_(s) configured to calculate an activecurrent instruction value i_(rsq)* based on a DC-side voltage set valuesignal v_(rdc)* of the reactive compensation device and a DC-sidevoltage signal v_(rdc) of the reactive compensation device obtained bysampling.

The seventh calculation module i_(s) configured to calculate a harmoniccurrent component i_(sh)* based on the voltage signal v_(s) of the powergrid and the current signal i_(s) of the power grid obtained bysampling.

The eighth calculation module i_(s) configured to calculate, based onthe reactive compensation current component i_(sq)* , a reactive currentinstruction value which is equal in size and opposite in direction tothe reactive compensation current component i_(sq)* , calculate, basedon the harmonic current component i_(sh)*, the reactive currentinstruction value, the active current instruction value i_(rsq)* and acurrent signal i_(rs) of the reactive compensation device obtained bysampling, a second modulation signal for causing an output power factorof the cascaded multi-level inverter system to be 1 and compensatingtotal harmonic distortion, and output the second modulation signalv_(r)* to the reactive compensation device.

The specific operating principle of the controller is the same to theforegoing embodiments, which will not be described in detail herein forsimplicity.

A cascaded multi-level inverter system is further provided in anotherembodiment of the present disclosure, as shown in FIG. 2. The systemincludes a reactive compensation device, multiple inverter units (takingan H-bridge module as an example in FIG. 2), a filter capacitor C, afilter inductor L and a controller. The controller adopts the modulationmethod for the cascaded multi-level inverter system described in anyembodiment above to realize power imbalance suppression, MPPTcontrolling of each DC source, an output power factor of 1 andcompensation of THD of an input current.

The reactive compensation device is connected with the power grid.

The multiple inverter units are connected with multiple DC sourcesrespectively.

Optionally, the reactive compensation device is a voltage-type reactivecompensation circuit, a current-type reactive compensation circuit or aswitch clamping type three-level reactive compensation circuit.

Referring to FIG. 8, the voltage-type reactive compensation circuitincludes an

H-bridge inverter module, a first capacitor C1 and a first inductor L1.

The first capacitor C1 is connected between two input terminals of theH-bridge inverter module.

One end of a first inductor L1 is connected with one output terminal ofthe H-bridge inverter module.

The other end of a first inductor L1 and the other output terminal ofthe H-bridge inverter module are connected with the power gridrespectively.

Referring to FIG. 9, the current-type reactive compensation circuitincludes an H-bridge inverter module and a second inductor L2.

The second inductor L2 is connected between two input terminals of theH-bridge inverter module

Two output terminals of the H-bridge inverter module are connected withthe power grid respectively.

Referring to FIG. 10, the voltage-type compensation circuit includes anH-bridge inverter module, a second capacitor C2, a third capacitor C3, afirst switching transistor S1, a second switching transistor S2 and athird inductor L3.

The second capacitor C2 and a third capacitor C3 are connected in seriesbetween two input terminals of the H-bridge inverter module.

The series connection point between a second capacitor C2 and a thirdcapacitor C3 is connected, through a first switching transistor S1 and asecond switching transistor S2 which are connected in anti-series, withone output terminal of the H-bridge inverter module and one end of athird inductor L3.

The other end of a third inductor L3 and the other output terminal ofthe H-bridge inverter module are connected with the power gridrespectively.

In practice, the reactive compensation device may be a bridge circuitcomposed of a semiconductor, a capacitor, a inductor and the like, suchas a voltage-type reactive compensation circuit (as shown in FIG. 8 as atypical example) and a current-type reactive compensation circuit (asshown in FIG. 9 as a typical example). Of course, the reactivecompensation device may also be combined with multi-level technology toform all kinds of derivative circuits, such as a switch clamping typethree-level reactive compensation circuit shown in FIG. 10. However,specific implementation forms of the reactive compensation device arenot limited to examples shown in FIG. 8 to FIG. 10, and may depend onspecific application environments, which all fall within the scope ofthe present disclosure.

The specific operating principle of the cascaded multi-level invertersystem is the same to the foregoing embodiments, which will not bedescribed in detail herein for simplicity.

What is described above is only the preferable embodiments of thedisclosure and is not intended to define the disclosure in any form.Though the disclosure is disclosed by way of preferred embodiments asdescribed above, those embodiments are not intended to limit thedisclosure. Numerous alternations, modifications, and equivalents can bemade to the technical solution of the disclosure by those skilled in theart in light of the technical content disclosed herein without deviationfrom the scope of the disclosure. Therefore, any alternations,modifications, and equivalents made to the embodiments above accordingto the technical essential of the disclosure without deviation from thescope of the disclosure should fall within the scope of protection ofthe disclosure.

The invention claimed is:
 1. A modulation method for a cascadedmulti-level inverter system, applied to a controller for the cascadedmulti-level inverter system, the cascaded multi-level inverter systemcomprising a reactive compensation device and a plurality of inverterunits, the reactive compensation device and the plurality of inverterunits being connected with the controller, the reactive compensationdevice being connected with a power grid, the plurality of inverterunits being connected with a plurality of DC sources respectively, themodulation method comprising: performing a maximum power point trackingcontrol based on a voltage signal and a current signal of each of theplurality of DC sources and a voltage signal and a current signal of thepower grid obtained by sampling, calculating a first modulation signalfor suppressing power imbalance, and outputting the first modulationsignal to each of the plurality of inverter units; calculating areactive compensation current component based on the voltage signal andthe current signal of each of the plurality of DC sources and thevoltage signal of the power grid obtained by sampling; calculating areactive current instruction value which is equal in size and oppositein direction to the reactive compensation current component based on thereactive compensation current component; calculating an active currentinstruction value based on a DC-side voltage set value signal of thereactive compensation device, and a DC-side voltage signal of thereactive compensation device obtained by sampling; and calculating,based on the reactive current instruction value, the active currentinstruction value, and a current signal of the reactive compensationdevice obtained by sampling, a second modulation signal for causing anoutput power factor of the cascaded multi-level inverter system to be 1,and outputting the second modulation signal to the reactive compensationdevice.
 2. The modulation method for the cascaded multi-level invertersystem according to claim 1, wherein before the process of outputtingthe second modulation signal to the reactive compensation device, themethod further comprises: calculating a harmonic current component basedon the voltage signal and the current signal of the power grid obtainedby sampling; and calculating, as an output, a second modulation signalfor compensating total harmonic distortion, based on the harmoniccurrent component and the second modulation signal for causing an outputpower factor of the cascaded multi-level inverter system to be
 1. 3. Themodulation method for the cascaded multi-level inverter system accordingto claim 1, wherein the process of performing a maximum power pointtracking control based on a voltage signal and a current signal of eachof the plurality of DC sources and a voltage signal and a current signalof the power grid obtained by sampling and calculating a firstmodulation signal for suppressing power imbalance, comprises: performinga maximum power point tracking calculation and a maximum power pointtracking control based on the voltage signal and the current signal ofeach of the plurality of DC sources obtained by sampling, and acquiringa power instruction value of each of the plurality of inverter units;calculating a power gird current instruction value based on the powerinstruction value of each of the plurality of inverter units, thereactive compensation current component, and the voltage signal of thepower grid obtained by sampling; calculating a modulation voltageinstruction value based on the power grid current instruction value, andthe current signal of the power grid obtained by sampling; dividing themodulation voltage instruction value into an active modulation signaland a reactive modulation signal; and distributing the active modulationsignal and the reactive modulation signal according to an active powerdistribution principle and a reactive power distribution principlerespectively, and calculating, by means of vector synthesis, the firstmodulation signal to be outputted to each of the plurality of inverterunits.
 4. A cascaded multi-level inverter system, comprising a reactivecompensation device, a plurality of inverter units, and a controller inwhich the modulation method for the cascaded multi-level inverter systemaccording to claim 1 is applied, wherein the reactive compensationdevice is connected with a power grid; the plurality of inverter unitsare connected with a plurality of DC sources respectively; and thereactive compensation device is a voltage-type reactive compensationcircuit, a current-type reactive compensation circuit or a switchclamping type three-level reactive compensation circuit.
 5. The cascadedmulti-level inverter system according to claim 4, wherein thevoltage-type reactive compensation circuit comprises an H-bridgeinverter module, a first capacitor and a first inductor; wherein thefirst capacitor is connected between two input terminals of the H-bridgeinverter module; one end of the first inductor is connected with oneoutput terminal of the H-bridge inverter module; and the other end ofthe first inductor and the other output terminal of the H-bridgeinverter module are connected with the power grid respectively.
 6. Thecascaded multi-level inverter system according to claim 4, wherein thecurrent-type reactive compensation circuit comprises an H-bridgeinverter module and a second inductor; wherein the second inductor isconnected between two input terminals of the H-bridge inverter module;and two output terminals of the H-bridge inverter module are connectedwith the power grid respectively.
 7. The cascaded multi-level invertersystem according to claim 4, wherein the voltage-type reactivecompensation circuit comprises an H-bridge inverter module, a secondcapacitor, a third capacitor, a first switching transistor, a secondswitching transistor and a third inductor; wherein the second capacitorand the third capacitor are connected in series between two inputterminals of the H-bridge inverter module; the series connection pointbetween the second capacitor and the third capacitor is connected,through the first switching transistor and the second switchingtransistor which are connected in anti-series, with one output terminalof the H-bridge inverter module and one end of the third inductor; andthe other end of the third inductor and the other output terminal of theH-bridge inverter module are connected with the power grid respectively.8. A controller for a cascaded multi-level inverter system, the cascadedmulti-level inverter system comprising a reactive compensation deviceand a plurality of inverter units, the controller being applied to thereactive compensation device and the plurality of inverter units, thereactive compensation device being connected with a power grid, theplurality of inverter units being connected with a plurality of DCsources respectively, the controller comprising: a first modulationmodule configured to perform a maximum power point tracking controlbased on a voltage signal and a current signal of each of the pluralityof DC sources and a voltage signal and a current signal of the powergrid obtained by sampling, calculate a first modulation signal forsuppressing power imbalance, and output the first modulation signal toeach of the plurality of inverter units; and a second modulation moduleconfigured to calculate a reactive compensation current component basedon the voltage signal and the current signal of each of the plurality ofDC sources and the voltage signal of the power grid obtained bysampling; calculate a reactive current instruction value which is equalin size and opposite in direction to the reactive compensation currentcomponent based on the reactive compensation current component;calculate an active current instruction value based on a DC-side voltageset value signal of the reactive compensation device, and a DC-sidevoltage signal of the reactive compensation device obtained by sampling;and calculate, based on the reactive current instruction value, theactive current instruction value, and a current signal of the reactivecompensation device obtained by sampling, a second modulation signal forcausing an output power factor of the cascaded multi-level invertersystem to be 1, and output the second modulation signal to the reactivecompensation device.
 9. The controller for the cascaded multi-levelinverter system according to claim 8, wherein the second modulationmodule is further configured to: calculate a harmonic current componentbased on the voltage signal and the current signal of the power gridobtained by sampling; and calculate, as an output, a second modulationsignal for compensating total harmonic distortion, based on the harmoniccurrent component and the second modulation signal for causing an outputpower factor of the cascaded multi-level inverter system to be
 1. 10.The controller for the cascaded multi-level inverter system according toclaim 8, wherein the first modulation module comprises: a firstcontrolling module configured to perform a maximum power point trackingcalculation and a maximum power point tracking control based on thevoltage signal and the current signal of each of the plurality of DCsources obtained by sampling, and acquire a power instruction value ofeach of the plurality of inverter units; a first calculation moduleconfigured to calculate a power gird current instruction value based onthe power instruction value of each of the plurality of inverter units,the reactive compensation current component, and the voltage signal ofthe power grid obtained by sampling; a second calculation moduleconfigured to calculate a modulation voltage instruction value based onthe power grid current instruction value, and the current signal of thepower grid obtained by sampling; a third calculation module configuredto divide the modulation voltage instruction value into an activemodulation signal and a reactive modulation signal; and a fourthcalculation module configured to distribute the active modulation signaland the reactive modulation signal according to an active powerdistribution principle and a reactive power distribution principlerespectively, and calculate, by means of vector synthesis, the firstmodulation signal to be outputted to each of the plurality of inverterunits.
 11. The controller for the cascaded multi-level inverter systemaccording to claim 9, wherein the first modulation module comprises: afirst controlling module configured to perform a maximum power pointtracking calculation and a maximum power point tracking control based onthe voltage signal and the current signal of each of the plurality of DCsources obtained by sampling, and acquire a power instruction value ofeach of the plurality of inverter units; a first calculation moduleconfigured to calculate a power gird current instruction value based onthe power instruction value of each of the plurality of inverter units,the reactive compensation current component, and the voltage signal ofthe power grid obtained by sampling; a second calculation moduleconfigured to calculate a modulation voltage instruction value based onthe power grid current instruction value, and the current signal of thepower grid obtained by sampling; a third calculation module configuredto divide the modulation voltage instruction value into an activemodulation signal and a reactive modulation signal; and a fourthcalculation module configured to distribute the active modulation signaland the reactive modulation signal according to an active powerdistribution principle and a reactive power distribution principlerespectively, and calculate, by means of vector synthesis, the firstmodulation signal to be outputted to each of the plurality of inverterunits.